Process pumps can be found mounted in many ways. While there are situations in which a pump can be adequately mounted at least cost, there are also instances when process pumps need to be mounted with uncompromising diligence and quality of workmanship. It is therefore important to understand how a pump should be mounted.
By Heinz P. Bloch
Securing Pumps in Place
Proper field installation of pumps has a measurable positive impact on a pump’s life. Even a superb design will give poor results if poorly installed. Conversely, a moderately good pump design, properly installed, will give good results.
A pump can be considered to have been correctly mounted if the installation has a good foundation design, no pipe strain, and good shaft alignment. It must also be properly secured to its respective baseplates, and the baseplates must be well-bonded to the underlying foundation. Epoxy grout is often used to do this bonding in modern installations.
No pump manufacturer designs its pumps strong enough to act as a solid anchoring point for incorrectly supported piping, or piping that causes casings and pump nozzles to yield and deflect.
Figure 1 depicts an example of a poorly installed mount. There are obviously some flaws in the grout surrounding the base plate, such as the hollow space under the electric motor. Lack of support under motors often invites resonant vibration. Rigorous written installation procedures are therefore needed and must be adhered to if long equipment life is to be achieved. One exception to these parameters is the vertical in-line pump, Figure 2. Vertical in-line pumps are not to be bolted to the foundation, as they are intended to respond to thermal and other growths of the connected piping. They must therefore be allowed to float or slide a fraction
of an inch in the x and y-directions. The foundation mass required under vertical in-line pumps can be significantly less than that found under the more typical horizontal pump.
Making the foundation mass three to five times the mass of the pump and its driver has been the rule of thumb for horizontal pumps. For vertical in-line pumps it is acceptable to make the concrete foundation about one-and-a-half to twice the mass of the pump-and-driver combination.
Additional Installation Considerations
To ensure level mounting following installation, the base plate is placed on a foundation into which hold-down bolts or anchor bolts (Figure 4) are encased when the reinforced concrete foundation is being poured. For proper stretch and long life, these anchor bolts must have a diameter-to-length ratio somewhere between 1:10 and 1:12. The anchor bolts are provided with sleeves and flexible fill. The sleeves prevent entry of grout and accommodate the differing amounts of thermal growth of a concrete foundation relative to that of a steel base plate.
It is important to note, that bespoke situations can occur. In Figure 4, for example, the purchaser of the pump specified an arrangement that allows insertion (and later removal) of alignment jacking tabs in the x and y-directions next to each of the four motor feet. Portable jacking tabs, Figure 4, (inserted in a welded-on bracket) allow driver alignment moves to be made. Thereafter, the jacking bolts are backed-off and the entire tab is removed.
When jack screws are left tightened against the motor feet, motor heat and thermal growth might force the feet into these bolts even more, sometimes causing the entire motor casing to distort. Note, therefore, that backing-off jacking bolts should be one of many items on an installation checklist.
Use of Stilt Mounts
Plants that use stilt mounting (Figure 5) often fall short of achieving best-possible equipment reliability. Best practices plants typically secure their pumps solidly on more traditional foundations. Although stilt-mounted pump sets lack overall stiffness, they have been used for small ANSI pumps where the sideways-move capability of the entire installation was thought to equalize piping-induced stresses. Among its few advantages are low initial cost and ease of cleanup; this is specifically true if ceramic floors are to be hosed down for hygiene reasons.
There are, however, serious shortcomings to this move-allowing mounting method. Stilt-mounting will not allow pump vibration to be transmitted through the baseplate to the foundation of the pump and then down through the subsoil.
Final Thoughts
Proper traditional foundation mounting permits transmission of vibration which can result in a significant increase in mean time between failures (MTBF), longer life of mechanical seals and bearings, and attractive total life cycle cost.
Heinz P. Bloch’s professional career commenced in 1962 and included long-term assignments as Exxon Chemical’s Regional Machinery Specialist for the United States. He has authored or co-written over 800 publications, among them 24 comprehensive books on practical machinery management, failure analysis, failure avoidance, compressors, steam turbines, pumps, oil mist lubrication and optimized lubrication for industry. Mr. Bloch held B.S. and M.S. degrees (cum laude) in Mechanical Engineering. He was an ASME Life Fellow and was awarded life-time registration as a Professional Engineer in New Jersey. He was one of 10 inaugural inductees into NCE’s Hall of Fame, which honors its most distinguished alumni.